1. Field of the Invention
The present invention relates to an outer race of a universal joint having a plurality of ball rolling grooves provided on the inner periphery of a cylindrical portion which is integral with a shaft, and a method for manufacturing the same.
2. Description of the Related Art
An outer race 1 of a universal joint having crossed grooves shown in FIG. 4 is made by forming a plurality of crossed ball rolling grooves 2 inclined by a predetermined angle with respect to the axial direction on the inner periphery of a cylindrical portion 1b which is integrally made with a shaft portion 1a, and assembling a cage and a plurality of balls not shown, thereby constituting a constant velocity universal joint.
The outer race 1 of the universal joint is fabricated by applying semi-hot forging to a blank thereby to perform preliminary forming of the material substantially into a predetermined shape except for the ball rolling grooves 2, then forming the ball rolling grooves 2 by applying the so-called ironing process which is a type of cold process. The ironing process is such that, as shown in FIG. 5, a punch set 4, which can be expanded and reduced in the radial direction and has a plurality of ridges 3 of a configuration complementary to that of the ball rolling grooves 2 to be formed on the inner periphery, is fitted into the inner periphery of the cylindrical portion 1b of the outer race 1 which is a blank having been pre-formed substantially to the predetermined shape except for the ball rolling grooves 2. Under this condition, the outer periphery of the cylindrical portion 1b is ironed with a die 6 having a tapered die hole 5, while pressing the punch set 4 against the inner periphery of the cylindrical portion 1b, thereby forming the ball rolling grooves 2 of the predetermined shape on the inner periphery of the cylindrical portion 1b. 
In the outer race 1 of the universal joint described above, the ball rolling grooves 2 are finished to the final dimensions by grinding, after forming the ball rolling grooves 2 in the intermediary configuration on the inner periphery of the cylindrical portion 1b by ironing process. For this purpose, in the prior art, a drill pocket which is a little deeper than the grinding allowance is machined at the base of each ball rolling groove 2 located at the innermost position in the inner periphery of the cylindrical portion 1b, as a margin recess for a grinding wheel for grinding the ball rolling grooves 2.
In the process of manufacturing the outer race 1 of the prior art, since each drill pocket is formed as a margin recess for a grinding wheel at the base of the ball rolling groove 2 by cutting operation after forming the ball rolling grooves 2 on the inner periphery of the cylindrical portion 1b by the ironing process, it is difficult to form the drill pockets by cutting due to the difficulty in determining the angular position of the drill pockets with respect to the ball rolling grooves 2 when forming the drill pockets by cutting.
Also because it is necessary to form the drill pockets in a separate machining process as a margin recess for a grinding wheel after forming the ball rolling grooves 2 on the inner periphery of the cylindrical portion 1b by ironing process, increased number of processes leads to a significant decrease in the productivity and an increase in the production cost due to the increased equipment cost.
Moreover, while each drill pocket which functions as a margin recess for a grinding wheel is formed by cutting, the cutting process tends to cause a margin recess for a grinding wheel having excessive depth to be formed and therefore leads to the possibility of such troubles as the ball falling into the drill pockets during assembly.
The present invention has been made for the purpose of solving the problems described above, and an object thereof is to provide an outer race of a constant velocity joint which has better operability and lower cost, and a method for manufacturing the same.
In order to achieve the object described above, the present invention provides an outer race of a universal joint, the outer race comprising a shaft portion and a cylindrical portion formed on one end of the shaft portion, fabricated by fitting a punch set having a plurality of ridges of a configuration complementary to that of ball rolling grooves to be formed on an inner periphery of the cylindrical portion into the inner periphery of the cylindrical portion of the outer race which is a blank, and forming the ball rolling grooves on the inner periphery of the cylindrical portion by ironing with the cylindrical portion being pressure-inserted into a die. The ironing operation is stopped when the ball rolling grooves of a predetermined length is formed at the base of the ball rolling grooves by ironing with the cylindrical portion inserted from the opening side thereof while leaving the inner-most portion of cylindrical portion intact, thus forming a margin recess for a grinding wheel.
Also the present invention provides a method for manufacturing an outer race of a universal joint, comprising the steps of providing an outer race which is a blank having a shaft portion and a cylindrical portion formed on one end of the shaft portion, fitting a punch set, which has a plurality of ridges of the configuration complementary to that of ball rolling grooves to be formed on the inner periphery of the cylindrical portion, into the inner periphery of the cylindrical portion of the outer race, and forming the ball rolling grooves on the inner periphery of the cylindrical portion by ironing with the cylindrical portion being pressure-inserted into the die. The method further comprises the steps of fitting the punch set into the inner periphery of the cylindrical portion with an initial clearance secured therebetween in the radial direction, forming the ball rolling grooves on the inner periphery of the cylindrical portion by ironing with the cylindrical portion being pressure-inserted into the die from the opening side thereof, and stopping the ironing operation when the ball rolling grooves of a predetermined length is formed while leaving an unformed portion at the base of the ball rolling grooves, thereby forming the margin recess for a grinding wheel.
According to the present invention, since the ironing operation is stopped when the ball rolling grooves of the predetermined length is formed by ironing of the cylindrical portion of the outer race on the opening side thereof thereby forming the margin recess for a grinding wheel at the base of the ball rolling grooves while leaving the inner-most portion of cylindrical portion intact, it is made possible to eliminate the separate process of forming the margin recess for a grinding wheel by cutting operation which is employed in the prior art.
These and other objects and advantages of the present invention will become clear from the following description with reference to the accompanying drawings